A typical backhoe includes a frame which may be mounted on the rear of a tractor or like implement, the frame supporting a boom assembly for pivotal movement with respect to the frame about a vertical axis. The boom assembly includes a bucket or other material-handling device for work operations, with articulation of the boom assembly by the backhoe operator providing material-handling in the desired fashion.
Backhoes and like excavators are used for a wide variety of material handling and excavation operations, and the business is highly competitive in nature. It is necessary that work operations be efficiently performed with the backhoe, and thus any way in which to shorten the cycle of filling the bucket, moving the boom assembly, dumping the bucket, and returning to repeat the cycle is very desirable.
In early backhoe designs, it was typically the practice of the backhoe operator to swing the boom assembly of the backhoe hard against the mechanical travel stops limiting the pivotal movement of the boom assembly. While this practice helped to reduce the time for each work cycle of the machine, this practice was found to be very detrimental to the components of the unit. If the operator exercised greater care during operation by manipulating the boom swing control to slow the assembly before it reached the travel stop, shock loading of the machine's components was reduced, but the extra care exercised by the operator increased the time for completing each cycle of backhoe operation.
In order to alleviate the problems caused by shock loading of the backhoe as the boom assembly contacted its mechanical travel stop, various arrangements have been employed to provide hydraulic cushioning of the boom assembly as it approaches the ends of its arc of travel. One type of arrangement is commonly referred to as a "stinger," which comprises a projection carried by the piston of each hydraulic motor provided for swinging the boom assembly. As the boom assembly of the backhoe approaches the ends of its arc of travel, the stinger of one of the hydraulic motors acts to restrict the flow of hydraulic fluid from that motor, thus providing hydraulic cushioning of the boom assembly. Frequently, a stinger arrangement is used in conjunction with a flow restricting orifice positioned in each motor which restricts hydraulic fluid flow from the motors to hydraulically cushion the boom assembly. While combination stinger/orifice cushioning arrangements are widely used today, their fabrication and maintenance has proven to be relatively expensive.
Another problem of conventional backhoe design relates to the relative positioning of the hydraulic motors of the boom swing mechanism and the resultant torque applied to the boom assembly. Spacial limitations are a major consideration in positioning the hydraulic motors of the swing mechanism, so the motors are usually positioned generally adjacent each other, and pivotally interconnected between the frame of the backhoe and the boom assembly. In order to obtain the desired range of travel for the boom assembly (approximately 180 degrees), this configuration of swing mechanism results in one or the other of the hydraulic motors moving through a fully extended position when the boom assembly is moved beyond either end of a central range of travel of approximately 90 degrees. This fully extended position is frequently referred to as the "center" position for that hydraulic motor. As the boom assembly is moved toward the ends of its arc of travel, one of the hydraulic motors moves through its fully extended center position and goes "overcenter", and then begins to contract. Each hydraulic motor of the swing mechanism moves through its center position when its centerline intersects the vertical swinging axis of the boom assembly.
As one of the hydraulic motors of the swing mechanism moves to and through its center position, the moment arm through which it acts upon the boom assembly approaches zero at the center position, with the overcenter hydraulic motor then exerting a negative torque on the boom assembly. Since the other (non-overcenter) hydraulic motor of the swing mechanism acts through a much greater moment arm than the overcenter motor, the boom assembly continues to move through its arc of travel. However, since the overcenter hydraulic motor exerts a negative torque on the boom assembly, the non-overcenter hydraulic motor must work to overcome this negative torque as it provides the primary force for pivoting the boom assembly. The negative torque created by the overcenter one of the hydraulic motors is particularly a problem when swinging movement of the boom assembly away from one of its travel stops is initiated, since inertial forces must be overcome. In this regard, a hydraulic swing mechanism for the boom assembly which includes an arrangement for redirecting the flow of hydraulic fluid to the hydraulic motors during swinging movement of the boom assembly so that the hydraulic motors are not, in essence, acting against each other, provides a more efficient swing mechanism with improved control for the backhoe operator.
Thus, a hydraulic swing mechanism for a boom assembly of a backhoe or like implement which provides improved hydraulic cushioning for the boom assembly, as well as improves the torque characteristics of the arrangement, further enhances the versatility and desirability of these types of material handling implements.